Patent Literature 1 describes an acoustic device, such as a mobile phone, that transmits air conduction sound and bone conduction sound to a user. Patent Literature 1 also describes that the air conduction sound is sound perceived by the auditory nerve of a user as a result of the eardrum being vibrated by air vibrations, created by a vibration of an object, that travel through the external auditory canal down to the eardrum. Furthermore, Patent Literature 1 describes that the bone conduction sound is transmitted to the auditory nerve of the user through a part of a user's body (e.g. a cartilaginous portion of the external ear) that is in contact with the vibrating object.
In the telephone described in Patent Literature 1, a rectangular plate-shaped vibration body configured by piezoelectric bimorph and a flexible material is attached to an outer surface of a housing via an elastic member. Patent Literature 1 also describes that, upon application of a voltage to the piezoelectric bimorph of the vibration body, the piezoelectric material is expanded and contracted in a longitudinal direction, thereby causing the vibration body to undergo flexture vibration. As a result, when the user places the vibration body in contact with the auricle, the air conduction sound and the bone conduction sound are transmitted to the user.
Apart from a telephone that transmits sound by being held in the hand and pressed against the ear, examples of other devices that transmit sound based on such a transmission principle include cartilage transmission earphones and headphones that are used by being hooked and held somewhere on the human head.